Avalanche photodiode (APD) structures that have separate absorption and multiplication layers (SAM-APDs) can provide electrical output signals with high fidelity (i.e., low noise). In a SAM-APD, the optical energy is absorbed and converted into electrical carriers in a layer specifically designed for efficient absorption (hereinafter, referred to as the “absorption layer”). The resulting electrical signal is amplified in a different layer specifically designed for efficient electrical carrier multiplication (hereinafter, referred to as the “cap layer”). By separating the absorption and multiplication functions into different semiconductor layers, each can be independently optimized for its intended purpose.
A device region can be formed in the cap layer of a SAM-APD by diffusion of a dopant into the semiconductor layer to form a p-n junction. The undoped portion of the cap layer that resides beneath the p-n junction provides a high-field region in which avalanche multiplication can occur (i.e., the avalanche multiplication region).
The principal driver for improved APD performance has been its use in telecommunications systems. For these applications, the APD is electrically biased such that the electrical response is substantially linear with optical power. Recently, interest has arisen in the use of APDs for detection of single photons in such applications as cryptography. For single-photon-detection applications, the APD is electrically biased at or beyond its “electrical breakdown voltage.” The breakdown voltage is the voltage at which the p-n junction is sufficiently reverse-biased to conduct a large current arising from a self-sustaining avalanche process—even in the absence of continuous optical power. An APD that is biased at or above breakdown, therefore, can give rise to an easily detectable pulse of electrical current in response to the absorption of even a single photon.
Two important parameters for an APD are the uniformity of the gain and breakdown voltage across the device region. Gain and breakdown voltage are functions of the thickness of the undoped portion of the device region. Dopant diffusion in a semiconductor is a substantially isotropic process (i.e., the dopant diffuses laterally and vertically, at nearly the same rate). As a result, it is well understood that a diffused p-n junction will have a central portion (hereinafter, referred to as the “active region”) and an outer portion (hereinafter, referred to as the “edge region”).
The active region is characterized by a uniform, planar junction profile while the edge region has a non-uniform, curved junction profile. In the active region, the uniform junction profile leads to uniform gain and uniform breakdown voltage. The curvature of the junction profile in the edge region, however, leads to a larger local electric field and therefore higher gain and lower breakdown voltage than in the active region. This undesirable phenomenon is typically referred to as “edge breakdown”. For practical SAM-APDs, the breakdown-voltage uniformity across the entire device region should be within 10%, and preferably within 1%.
Another important performance metric for an APD, particularly in a single-photon detection application, is Noise Equivalent Power (NEP). NEP is a function of the ratio of erroneous signals (referred to as the dark count rate) to optical detection efficiency. A photodiode with low NEP will contribute few false counts while still detecting most or all of the received photons.
A low NEP can be achieved by 1) high detection efficiency and/or 2) low dark count rate. Detection efficiency is a function of several factors: (i) the amount of the light signal which is directed into the detector (i.e., optical coupling efficiency); (ii) the probability that a received photon is absorbed by the detector (i.e., quantum efficiency); and (iii) the probability that the absorbed photon will result in a detectable avalanche event (i.e., avalanche probability).
A high coupling efficiency can be achieved by making the device region of an APD at least as large as the mode-field diameter of the optical beam. Many prior art photodiodes, in fact, have a device region that is larger than the mode-field diameter so as to both capture as much of the light as possible and allow for some misalignment while still capturing the entire beam.
Avalanche probability can be improved by increasing the bias voltage so that it is well above the breakdown voltage. The larger this overbias, the greater the probability that a received photon will generate an avalanche event. Unfortunately, dark count rate also increases with overbias; therefore in many cases increased overbias actually degrades NEP rather than improves it.
Device technologists in the communications field have long understood that device performance and manufacturing yield of semiconductor devices are functions of material quality. In the past few decades, therefore, effort has been directed toward improving crystal growth techniques so as to reduce semiconductor material defect density. Improved crystal growth techniques can also reduce the presence of defects that serve as nucleation sites for dark current mechanisms. However, radical improvements in the overall materials growth technology area would be required to affect any significant reduction of dark count rate. Moreover, it is unlikely that defects will ever be completely eliminated through improved growth technique in a cost-effective manner.
It is desirable, therefore, to develop an avalanche photodiode with improved NEP in a manner that is compatible with conventional crystal growth techniques and overcomes some of the costs and limitations of the prior art.